The development of small antennas for mobile phones has recently received much attention due to size reduction of the handsets, requirements to keep the amount of radio-frequency (RF) power absorbed by a user below a certain level regardless of the handset size, and introduction of multi-mode phones. It would be advantageous, desirable and even necessary to provide internal multi-band antennas to be disposed inside a handset body, and these antennas should be capable of operating in multiple band systems such as GSM850 (824 MHz–894 MHz) E-GMS900 (880 MHz–960 MHz), GSM1800 (1710 MHz–1880 MHz), and PCS1900 (1850 MHz–1990 MHz). Shorted patch antennas, or planar inverted-F antennas (PIFAs), have been used to provide two or more resonance frequencies. For example, Liu et al. (Dual-frequency planar inverted-F antenna, IEEE Transaction on Antennas and Propagation, Vol.45, No.10, October 1997, pp. 1451–1458) discloses a dual-band PIFA; Pankinaho (U.S. Pat. No. 6,140,966) discloses a double-resonance antenna structure for several frequency ranges, which can be used as an internal antenna for a mobile phone; Isohatala et al. (EP 0997 970 A1) discloses a planar antenna having a relatively low specific absorption rate (SAR) value; Ollikainen et al. “Internal Dual-band Patch Antenna for Mobile Phones, Proceedings AP2000 Millennium Conference on Antennas and Propagation” presented at Davos, Switzerland, Apr. 9–14, 2000, discloses a PIFA having resonance frequencies at E-GSM900, GSM1800 and PCS1900 bands, wherein one of the shorted patches is folded to provide a capacitive load to the E-GSM900 shorted patch; and Song et al. (Triple-band planar inverted-F antenna, IEEE Antennas and Propagation International Symposium Digest, Vol.2, Orlando, Fla., Jul. 11–16, 1999, pp. 908–911) discloses a triple-band PIFA.
Currently, quad-band (GSM 850/900/1800/1900) engines are already available for mobile phones, but the antenna is still an issue because it is one of the largest parts in a mobile phone. In order to fit more antenna elements with acceptable performance in the available space, there is an ongoing effort to reduce their physical size. With the constraints in physical size, existing internal multi-band antennas do not cover all of the GSM850, GSM900, GSM1800 and GSM1900 bands.